This invention relates to power semiconductors and in particular to circuitry for extending the range of anode currents of a four-layer insulated-gate-controlled device over which control can be maintained by gate-cathode potentials.
Four-layer insulated-gate-controlled devices or conductivity-modulated-FET's are devices which are fabricated similarly to MOS-gated thyristors but which include a shunting resistance between the base and emitter of the npn transistor. This shunting resistor has the effect of reducing the gain of the npn transistor and prevents the conductivity-modulated-FET from going into a latched stated over a wide range of anode current. Within this wide range, by the potential applied to the gate of the FET can control the current flowing between the anode-and-cathode and the anode current may be extinguished by applying appropriate turn-off gate potential. For anode currents exceeding this range, the device goes into a latched state which can only be overcome by direct reduction of the anode current. That is, the anode-to-cathode current is no longer a function of, or controlled by, the gate potential, and a turn-off potential applied to the gate can not extinguish or interrupt the anode-to-cathode current.
A problem exists in that devices operated within a supposedly controllable (i.e. linear and non-latched) region assume, or go into, a latched condition (i.e. a condition where the gate-to-cathode potential has no control over the anode-to-cathode current level and cannot cut-off conduction) when a relatively sharp turn-off signal is applied to their gate terminals.